During the last year, we continued to define the molecular events that regulate hematopoietic stem and progenitor cell (HSPC) quiescence, survival, self-renewal and, myeloid cell lineage commitment and differentiation. We have focused our efforts on transcription factors since they are essential for stem cell growth and differentiation, and are frequently deregulated during the development of leukemias and lymphomas. We previously found that the helix loop helix (HLH) transcription factor, inhibitor of differentiation 1 (Id1), is induced during the early stages of myeloid development, and can instruct hematopoietic stem cells toward a myeloid versus erythroid and lymphoid cell fate suggesting that this gene and other family members Id2 and Id3 may regulate cell specification from HSPC. We have also found that Id1 is required for normal hematopoietic development using Id1-/- mice. Id-/- mice are viable and show no obvious defects. However, these mice show impaired hematopoietic development including increased hematopoietic stem/progenitor cell cycling, and defects in B cell and myeloid cell development. We determined that the hematopoietic phenotype observed in the Id1-/- mice was not intrinsic to the Id1-/- hematopoietic cells, but was due to defects in the microenvironment or niche. The hematopoietic microenvironment is a complex mixture of cells that includes endothelial cells, mesenchymal stem and progenitor cells and their differentiated progeny, adipocytes, osteoblasts, smooth muscle, and chondrocytes. Therefore, we developed and tested a conditional mouse model of Id1, and have initiated studies to specifically delete Id1 in endothelial cells, osteoblasts and other stromal cell populations using an Id1 conditional mouse model recently developed in our lab, and transgenic mouse strains that express cre recombinase in endothelial, adipocytes and osteoblasts cell lineages. Since Id genes are known to compensate for each other, we evaluated if Id3 compensates for loss of Id1 by deleting both genes intrinsically and extrinsically using conditional mouse models and transgenic mice that express cre in specific cell lineages. We found that deletion of Id1 and Id3 in the hematopoietic microenvironment lead reduced overall hematopoietic output and extra medullary hematopoiesis, while BMC that lacked Id1 and Id3 developed normally when transplanted into recipient mice. Unexpectedly, we discovered that mice that lacked Id1 or Id1 and Id3 showed enhanced HSC self-renewal compared to WT BMC. We found that the Id1-/- HSCs show reduced cell cycling and reduced levels of ?H2AX staining suggesting that the Id1-/- HSC show increased quiescence during BMT. We also found that the Id1-/- HSCs show reduced mitochondrial biogenesis, reduced ROS production and increased levels of reduced glutathione confirming that Id1-/- HSCs show increased quiescence after BMT. Additional studies demonstrate that Id1-/- HSCs show reduced proliferative response to cytokines in vitro. Id1 is induced in HSCs by cytokines in vitro and in vivo and promote HSC proliferation. Collectively, we provide evidence that Id1-/- HSCs are protected from proliferative exhaustion during BMT, chronic stress and aging. Current studies are pursing novel methods and approaches to reduce Id1 expression in HSCs in vivo to protect HSCs from exhaustion during stress and aging. We previously discovered that Id2 is a physiological regulator of B cell and erythroid cell development by negatively regulating functions of E2A and Pu.1 respectively. We have recently discovered that Id2 is required to maintain HSCs during steady state, and are pursing mechanistic experiments to understand the downstream pathways and targets of Id2 action. To identify novel transcriptional regulators of myeloid cell growth and differentiation, we have compared the global gene expression profile of undifferentiated and differentiating hematopoietic progenitor cells. We have identified a novel zinc finger transcription factor of unknown function, POGZ, which is down regulated during the early stages of myeloid development. We have generated a mouse strain with a targeted deletion of POGZ. POGZ-/- mice do not survive beyond the first few hours of life, and die at birth of unknown causes, suggesting that POGZ is essential for mouse survival. We have discovered that fetal liver hematopoietic cell development is impaired in POGZ-/- mice, which includes a dramatic decrease in cellularity. We have found that loss of Pogz expression leads to a decrease in Bcl11a and KLF2 expression, and that embryonic globin genes are not silenced. We have found that Pogz regulates embryonic globin expression, in part, by regulating Bcl11a gene expression via a recently discovered erythroid specific enhancer element in the second intron of Bcl11a, and that POGZ is bound to the Bcl11a enhancers using ChIP assays. We have found that fetal globin expression is not repressed in mice transplanted with Pogz knockout fetal liver cells, and have recently demonstrated that POGZ is intrinsically required to repress fetal globin genes in vivo using the Mx1-cre transgenic mouse model.